1. Field of the Invention
The present invention relates to a premix burner.
2. Discussion of Background
Lean premixed combustion is a common method of achieving low pollutant emissions, in particular nitric-oxide emissions, in the combustion of fuels having a low content of nitric-oxide compounds. It has become known from publications that a further reduction in the nitric-oxide emissions, in particular during combustion at high pressure, as is the case in the modern generation of gas turbines, is possible with experimental burners by improving the mixture quality of air and fuel. However, applying such experimental burners to machine technology is not readily possible, since there are stringent requirements here with regard to flame stabilization and flashback safety. Conventional swirl-stabilized premix burners suitable for machines intermix the fuel with the combustion air only just before the flame zone.
Investigations in this connection have shown that homogeneous mixing of air and fuel up to the flame zone still cannot be achieved in this way. Shifting the fuel injection upstream to prolong the mixing time and thus improve the mixing quality is not permitted in a burner suitable for machines on account of the flashback risk associated therewith.
WO 93/17279 has disclosed a burner which essentially comprises a cylindrical chamber, which in turn has a plurality of tangentially arranged slots through which the combustion air flows into the interior of the chamber. In the region of these slots, at the transition to the interior space of the chamber, a number of fuel nozzles act in axial direction, through which preferably a gaseous fuel is admixed with the combustion air flowing through there. Furthermore, the interior space of the chamber is provided with a conical body, which tapers in the direction of flow, further fuel nozzles for a preferably liquid fuel being provided in the region of the tip of this conical body. The combustion air is made to ignite downstream of the cone tip of this body. In order to keep the flame stable outside the premix section of the burner, the flow in the chamber or premix section itself must be supercritical, i.e. the swirl coefficient must be so small here that no vortex breakdown occurs. The critical swirl coefficient can be achieved at the correct location by three paramaters: by a change in the width of the tangential slots, and on the other hand by an adaptation of the angle of the conical body in the interior space of the chamber, and also by the addition of central assisting air, whether swirled or without a swirl. Due to the fuel injection in the region of the slots, however, the latter are greatly restricted in their design. In addition, optimum homogeneous mixing of air and fuel cannot be achieved directly; this applies in particular to those fuel injections which are located at the end of the burner and which are therefore located directly in the region of the flame front, whereby there is also a potential flashback risk due to this proximity. Furthermore, both the gaseous fuel and the liquid fuel will not be readily mixed with the air due to the short distance from injection up to the flame, a factor which results in local rich zones in the flame, which lead to high NOx emissions and higher pulsations.
In summary, the following problems occur in such a burner:
a) increase in the risk of flashback of the flame, PA1 b) smaller operating range with optimum flame position, PA1 c) the NOx emissions increase, PA1 d) high pulsations, PA1 e) inadequate burn-out.